Enumeration of the antigenic determinants of human secretory component (SC) and secretory IgA (sIgA).

Abstract The number of antigenic determinants of human SC and sIgA was determined with specific anti-SC antibodies isolated from a pool of two hyperimmunized sheep. The Fab' antibody fragments were prepared and [ 125 I]labelled, while pure SC and pure sIgA, isolated from colostrum, were labelled with 131 I. The [ 125 I]Fab' antibodies were added, in very large molar excess, to the [ 131 I]antigens at various ratios. The Fab'—Ag complexes were separated from the antibody excess by gel-filtration. The highest Fab'/Ag molar ratios of the complexes, which correspond to the maximal number of accessible antigenic determinants, were calculated. We found at least 16 sites (16.6 ± 1.36) on free SC while only 12 (12.27 ± 0.51) were located on sIgA. These results confirm the existence of a significant number of hidden determinants of the SC subunit of sIgA and establish that about 1 4 of the SC molecule is implied in this binding.